Aggression and anxiety in adult with downsyndrome

My younger brother have down syndrome , he is 24 years old right now .from past 3-4 years , he has been showing aggression and anxiety and during these episodes he hit my parents. He doesn't intentionally try to harm my parents it's just when he is overwhelmed with those emotions he is Just not able to control himself. We try to avoid triggers as much as possible. But sometimes things are Just out of control . For example, now adays he has developed anxiety of rainy weather . So whenever the weather becomes cloudy and winds starts then he get anxious becomes aggressive , starts vomiting, We try to close curtains so that he can't see outside but he don't want that he wants to see what's happening outside but also get scared in that weather. Another example is that...my parents can't talk to anyone else, he gets aggressive when someone comes to our house and my parents talks to them. He also don't like people coming over the house , he doesn't like people with loud voices, doesn't likewhen my parents laught with someone or like spending Good time with anyone else....even sometimes with me as well .Just for the context, I am married and obviously have moved out. So as now I visit after sometimes so he doesn't get aggressive at me.. but I also know he don't want to hurt my parents I can see that he can't control his feeling he gets scared and anxious and in the heat of emotions he just hits them.

Now the problem is he can't even talk so he can not express himself fully ...so even if we take him to psychiatrist he won't be able to tell what's wrong with him plus I don't know how will psychiatrist treat him. We don't want any harm to him. We will not be able to see him in any kind of pain or discomfort. I want solution , I don't what to do, me and my parents have no problem taking care of him for the rest of our lives...it's that his aggressiveness and anxiety is getting out of hand .....my parents life has become isolated the can't go anywhere specially my mother.

I no longer think straight ....I am just here to ask if anyone know what the solution can be or just share something which might be helpful for me.

BTW sorry for bad english and grammar mistakes, english is not my first language and right now I am just so stressed that I don't even have energy to go back and correct any mistakes.